<!DOCTYPE html>
<html>
    <head>
        <title>Hidden overflow on HTML, hidden overflow on BODY</title>
    </head>
    <body style="background:white; color:black;">
        <div style="overflow-y:auto; overflow-x:hidden; width:20em; height:20em; background:cyan;">
            <p>This text should be inside a vertically scrollable cyan box.</p>
            <p>The viewport should not have scrollbars.</p>
            <div style="width:300vw; height:300vh;"></div>
        </div>
    </body>
</html>